1. Field of the Invention:
The present invention relates to crucibles for the thermal analysis of aluminum alloys, in particular alloys containing a hypereutectic amount of silicon.
2. Discussion of the Background
In cast components of aluminum alloys, one primarily uses alloys containing silicon. Among these alloys containing silicon, a distinction is made between alloys containing more than 12.5% by weight of silicon, which are referred to as hypereutectic, and alloys containing 12.5% or less by weight of silicon.
When hypereutectic alloys are cast, in the course of solidification they firstly give rise to the formation of crystals of silicon, referred to as primary. These crystals of silicon appear at a temperature referred to as the incipient solidification temperature. This formation of crystals of silicon is followed by the formation of eutectic crystals of aluminium-silicon.
The size of the crystals of primary silicon which are formed naturally is relatively large. This is harmful to the production of cast components having suitable properties. It is for that reason that in most cases a refining treatment of the alloy operation is carried out. This operation is carried out in the liquid state, and reduces the size of those crystals as much as possible.
This refining effect can be achieved for example by adding cupro-phosphorus in a variable amount depending on the proportion of silicon and other elements in the alloy being treated. However, that amount of refining agent may vary substantially for the same alloy depending on the treatments to which the metal has been subjected, the fusion procedure, and the presence of certain impurities.
It is for that reason that an excess of refining agent is generally added. Unfortunately, the effect of such a large addition is to dilute the alloy so that if the refining effect is correct, the silicon content of the alloy no longer corresponds to the desired composition.
Moreover, one skilled in this art is very well aware that refining agents have a fleeting effect. That is to say, frequently after just a few minutes between the moment at which the refining agent is introduced into the alloy and the moment of casting, the action of such an agent is reduced to zero.
In casting workshops, a ladle of alloy is used in most cases to successively feed a large number of moulds. With such an operation, if the refining effect is correct at the time of feeding the first moulds, it becomes inadequate and even non-existent for the later moulds.
It has therefore been found essential to be able to provide a way of being aware of the state of refining of the alloy bath at any moment in order to be able to provide for the addition of refining agent as may be necessary and thus avoid casting components not having the desired properties. Likewise, a method for measuring the precise proportion of silicon in the alloy diluted by the refining agent prior to the casting operation is necessary in order to be able to correct, if needed, the proportion of silicon in the alloy by adding a make-up amount of silicon.
Ways of determining the proportion of silicon in alloys are already found in all foundries. These methods involve the use of thermal analysis crucibles. Such crucibles are described in French Pat. No. 2 357 891 (=U.S. Pat. No. 4,105,191). They comprise a cylinder provided with a bottom. They are made either entirely of core sand or with an essentially metal side wall. Disposed within the cylinder is a sheath into which a thermocouple is inserted, the sensitive end of the thermocouple coming into contact with the alloy to be analysed when the alloy is poured in the liquid state into the crucible.
Such crucibles make it possible to follow variations in temperature during solidification of the alloy. In particular they make it possible to detect the incipient solidification temperature Tc, which is manifested by a change in the slope of the temperature-time curve.
In the case of hypereutectic aluminum-silicon alloys, that temperature Tc increases as a function of increasing proportions of silicon. It would thus seem to be an easy matter to determine the amount of silicon contained in the alloy analysed by comparison with previously established curves giving proportions of silicon as a function of the temperature Tc. Unfortunately, it has been found that the size of the primary silicon crystals also has an influence on Tc. Indeed, the larger the crystals, the lower the temperature Tc. So the amount of silicon contained in the alloy cannot be measured in this way.
As has been pointed out above, the degree of refining of industrial alloys may vary to a substantial extent, and likewise the proportion of silicon resulting from dilution by the refining agent may also vary considerably. Therefore, at the time of analysis knowledge of the temperature Tc will not make it possible to determine accurately the proportion of silicon contained in the alloy since that temperature Tc may result both from a combination of a substantial proportion of silicon and large crystals, and a combination of a small proportion of silicon and fine crystals.
There is thus a strongly felt need for a means permitting the suppression of the influence of the size of the crystals on Tc. Such a means would permit to determine with precision the amount of silicon present in the alloy, and at the same time permit to ascertain th degree of refining of the alloy being analysed.